This invention relates to the stoppage of water flow while permitting the recovery of hydrocarbons from a hydrocarbon formation in the earth.
Specifically, it relates to the placement of a filter/sieve of a blocking gel or polymer at a predetermined distance from a well bore in order to stop water flow and to thereby enhance the recovery of oil and gas hydrocarbons from the formation.
It is well known that the economic life expectancy of commercially productive oil and gas wells is determined by the transitional change with time from the well being predominantly oil and gas producing to increasingly becoming water productive. Under the best conditions the production of oil and gas only diminishes consistent with the depletion of the contained resource and at the uneconomic high water cut point the reservoir contains both non-produced mobile oil and gas and non-mobile residual oil and gas. At this high water cut point, the well is considered to be uneconomic for production of hydrocarbon from the specific perforated reservoir formation or interval and, as a consequence, production from that reservoir formation at that well location is abandoned. The quantity of residual oil remaining at this point however, is quite significant and residual oil saturation can range anywhere from 10 pore volume percent to in excess of 50 pore volume percent of the original oil or gas in place. This estimate does not take into account any volume of bypassed oil present in the reservoir.
The increasing production of water from a reservoir interval can also be attributed to other mechanisms such as water or gas coning, or early breakthrough of water or gas from high permeability zones present within the reservoir formation being produced.
Similar remarks apply to the injection of fluids into reservoir formations; the fluid flow profiles can be homogeneous or can be channeled into the formation by preferential flow through the higher permeability zones.
Blockage of high permeability zones within oil and gas productive reservoir has been commonly applied in the oil and gas industry as a means of reducing unwanted water and gas flow and improving oil and gas production. Both inorganic and organic gels and polymers have been used and there are a multitude of patents applicable to this type of blockage.
The common mode of operation is to inject into the well either preformed gels or polymer mixes or mixtures of chemicals which will interact at reservoir temperatures to form gels or polymer mixes with time. The ensuing plugging or blocking effects of these gels or polymers then inhibits the preferential production of water from the formation.
Problems most commonly experienced with the injection of preformed gels or polymer mixtures relate to inadequate depths of penetration into the formation followed by early breakdown of the blocking gel or polymer during the reverse production flow from the reservoir.
Injection of mixtures of gel or polymer forming chemicals with a secondary reactive chemical which induces gelation or polymer formation at depth in the reservoir suffers mainly in one being unable to control the reaction rate such that premature reaction does not occur prior to the chemical mixture being located at the desired depth of penetration. Premature gelation or polymerization of these chemical mixtures will often occur resulting in premature blockage at short distances (less than four feet) of penetration into the formation as is the case for direct gel or polymer injection. Formation of gels and polymers during the residence time spent by the chemical mixtures in the well bore during the injection is also a problem. Attempts have been made to diminish this effect by using coiled tubing to more speedily place the chemical mixes into the formation as well as using surfactant-emulsion transport of less water soluble and slower reacting acid forming chemicals.
Previous patent coverage relates to the use of Single Well Chemical Tracer technology for the measurement of residual oil saturation of watered-out reservoir formations (U.S. Pat. No. 3,623,842 (Nov. 30, 1971); Deans, H. A.: xe2x80x9cMethod for Determining Fluid Saturations in Reservoirs.xe2x80x9d) U.S. Pat. No. 4,312,635, issued to Carlisle on Jan. 26, 1982, provides background information for the determination of partition coefficients, which are discussed further below.
In Deans"" process, a volume of water (seawater, fresh water or formation water) containing a known concentration of reactive chemical tracer is injected into a watered out reservoir formation followed by the injection of a predetermined volume of water (push volume) such that the chemical tracer fluid volume is pushed into the reservoir to a desired distance. The reactive chemical tracer used is a chemical which has the ability to partition between the residual oil present as a stationary phase in the reservoir and the water phase which is moving through the reservoir consistent with the injection flow rate.
The partitioning effect between the reactive chemical and the stationary residual oil reduces the velocity of flow of the reactive chemical tracer bank relative to the water flow. Following injection of the chemical mix and the push volumes, the well is shut-in to allow the reactive chemical tracer to react with water to form a secondary nonreactive chemical product at the location of the reactive chemical tracer bank. Reaction time is controlled such that between 20-40 volume percent of the reactive tracer is converted to the secondary product tracer. Back production of the injected fluids and measurement of the returning reactive tracer and chemical product concentrations allows a determination of the accessible residual oil saturation (ASor) for the test interval to be made.
The preferred reactive chemical tracers used in the accessible residual oil saturation (ASor) measurement process are water soluble esters such as ethyl formate, methyl acetate, ethyl acetate among others. Hydrolysis of these chemicals under reservoir conditions form the corresponding acid and alcohol components making up that specific reactive tracer chemical. As a consequence of the acid formation, the hydrogen ion concentration or acidity (pH) will correspondingly increase.
Patents for the use of inorganic and organic gels and polymers as blocking agents in reservoir formations do exist. In most instances where inorganic gel chemicals have been used, the gel formation is initiated by mixing the gel progenitor chemical with inorganic acid or organic acid and alcohol chemicals. Chemical esters have been reported as a means of forming gels by the in situ generation of acid and alcohol components which correspondingly change the pH and initiate gelation or polymerization. However, such use of esters has only been applied to the mixing of the ester with the gel forming agent at the surface followed by co-injection of the chemicals into the reservoir.
This patent application relates to a process whereby a filter/sieve is produced by injecting the interactive chemicals used to form gels and polymers at reservoir temperatures independently and sequentially into a well in such a manner that the chemicals only come into contact with each other at the desired depth of penetration in the formation. At this location in the reservoir, which can be determined by appropriate calculation, the injection is stopped and the intermixed and superimposed chemicals are allowed to react to form the filter/sieve of a gel or polymer depending upon the nature of the individual chemicals injected.
Since no reaction takes place during the injection phase, premature gelation or polymerization cannot occur at any point other than where the chemicals come into contact each with the other. Furthermore, by using this placement process not only can the gel or polymer blockage, namely, the desired filter/sieve structure, be located at a depth of penetration (between four and thirty feet) where the velocity flow for either the injection or production of fluids into or from the reservoir interval is ideal for maintaining the blockage of water, but the thickness of the filter/sieve can also be predetermined by using appropriate volumes of the injected chemicals and nonchemical containing push volumes.